The use of natural gas as an energy source has become worldwide, and demand for the fuel is increasing. Natural gas wells now exist in large numbers in many locations across the earth and new wells are being found as a result of exploration activity that is increasing in this time of a growing energy shortage. Many natural gas wells are located close to existing or planned pipelines which, in most instances, provide an efficient and practical means to transport their natural gas from the well head to a terminal facility or place of use.
But a great many other natural gas wells are not presently found on or near a pipeline and, of these, many are so located that the economic and engineering problems associated with connecting them to a pipeline have the practical effect of precluding this occurring. These are referred to as shut-in gas wells, and there are several reasons why connecting them with a pipeline cannot be expected to occur.
Some shut-in gas wells are located where pipeline construction is very difficult and expensive, such as in deep water off shore. Others are scattered in small numbers across large geographic areas, and the amount of natural gas which can be recovered from them will simply not support the building of a pipeline. In other instances, it might prove economical to build a pipeline to the well head, if the amount of producible natural gas could be determined; but no practical, efficient method has been available in the past to determine expected production and thus such gas wells remain shut-in.
The amount of natural gas found in these shut-in gas wells is enormous, and efforts have been made to find a means of recovery that is at once practical and economical. But despite the work of many, the problem remained unsolved until the development of the invention entitled "Method and System for Transporting Natural Gas to a Pipeline", which is the subject of United States patent application, Ser. No. 912,853, filed June 5, 1978, now U.S. Pat. No. 4,139,019.
Until the invention which is described in this noted earlier application and patent, there were in use essentially only two techniques for transporting natural gas from the well head. The first was the pipeline, and its limitations have already been noted. The second was by a cryogenic technique in which the natural gas is refrigerated until it reaches a liquid state, and the liquefied natural gas is then placed within heavily insulated vessels which are transported under refrigerated conditions from the gas well location to an off-loading terminal. Examples of the second, liquefied natural gas transportation technique are described in U.S. Pat. Nos. 3,232,725 and 3,298,805. The liquefied natural gas transportation technique is very expensive to utilize, and requires the movement of very large volumes of natural gas from one location to another in order to be economically feasible. Further, because of economics and the heavy weight of the required equipment, the technique is usually only feasible when the vessels are mounted on ships for movement in water. Thus, the liquefied natural gas technique is simply not practical for use with most shut-in gas wells particularly those found in isolated, scattered locations on land.
In the method and apparatus of the invention of application Ser. No. 912,853, the natural gas is transported in discrete batches under high pressure within uninsulated and unrefrigerated, movable high pressure vessels, which can be carried by truck, train, water-borne craft or other suitable vehicles. The technique is practical from the standpoint of both engineering and economic feasibility for use with nearly all shut-in gas wells and thus for the first time makes available the vast natural gas reserves found in those gas wells.
The desire when taking natural gas from a gas well is of course to recover the maximum amount, and this becomes especially important when working with small numbers of isolated, shut-in gas wells of somewhat limited capacity. In the latter instances, the ability to recover the maximum amount of natural gas can make the difference between an economically successful project and one that might fail financially, and thus prove unfeasible. The present invention, which is an improvement on the invention of application Ser. No. 912,853 is particularly directed to assuring the maximum recovery of natural gas from a gas well.
It has been discovered that natural gas recovery can be hindered if the gas well is handled in a manner to shock the well. Well shock can occur from different causes and is generally defined for purposes of this invention as structural damage or problems occurring in the gas well during withdrawals therefrom, and which act to limit the recovery of natural gas therefrom.
It is known that natural gas produced from some underground reservoirs will have liquids associated therewith, which may be in the form of condensed hydrocarbon gases, called condensate, or water. Their presence can affect the flowing characteristics of the well, and it has been found preferable to transport the liquids to the surface by the natural gas flow. If the natural gas flow rate is not sufficient to lift the liquids out of the well, they can accumulate and impose an additional back pressure on the formation which can act to significantly affect the natural gas production capacity of the well or even render it incapable of production.
It is not uncommon for a gas well to begin production, but then to be affected by liquids in such a manner as to reduce or close down production. Obviously, when this type of well problem occurs, the natural gas recovery is affected. We have discovered this problem can be alleviated by carefully choosing the flow rate for the natural gas.
It has also been found that well shock can occur from changes in the velocity gradient in the well bore, which can cause a "sanding up" problem with the gas well. This form of well shock can be caused by high flow rates of gas, accompanied by intermittent flows, such as might result when large vessels are periodically filled from a gas well. The present invention addresses this problem, also.
Given the need for increased production of natural gas present in the world today, it is desirable to recover the maximum amount of natural gas from all gas wells. This is particularly important when working with shut-in wells, to be certain that the recovery operation is made economically feasible. There is thus need for an improved method and system for producing and transporting natural gas from shut-in wells, one which will minimize well shock, and the adverse effects thereof, and assure maximum natural gas recovery. The present invention is intended to satisfy that need.